Anxiety disorders are the most common mental illness in the United States. Current treatments include medications that alleviate the symptoms of the disorders but do not target the underlying cause. Attempts to treat the variety of symptoms that can arise in complex anxiety disorders with polypharmacy increase the potential for dangerous interactions. A promising alternative to medication in the treatment of anxiety disorders is Cognitive Behavioral Therapy (CBT). The aim of CBT is to rehabilitate, rather than to treat symptoms. CBT depends on the principle of fear extinction, which involves exposure to fear-associated cues until new associations are learned. Recent developments indicate that rehabilitation can be improved with adjunct therapies. Our research indicates that the vagus nerve plays a role in the consolidation of emotionally arousing memories. The vagus nerve serves as a bridge between the peripheral and central nervous systems, responding to increases in circulating stress hormones by increasing plasticity in the brain to enable rapid storage of memories of threatening or emotionally exciting events. The vagus nerve reciprocates sympathetic activation with parasympathetic effects on the heart and other organs. For this reason it is called the vagal brake on the sympathetic response to stress. Anxiety disorders are characterized by altered vagal responses to stress and impaired ability to extinguish conditioned fear. We recently discovered that stimulation of the vagus nerve during extinction training enhanced extinction of two forms of conditioned fear in rats. Vagus nerve stimulation (VNS) is approved by the Federal Food and Drug Administration for treatment of epileptic seizures and depression, but it is not currently administered as adjunct therapy. Instead, VNS is administered as a pacemaker to maintain a certain brain state. The proposed research is based on the concept that extinction failure leads to anxiety disorders and stimulation of the vagus nerve can enhance extinction. If VNS holds promise as a novel approach to the treatment of anxiety disorders, the mechanism/s of action should be better understood. The projection from the infralimbic cortex to the amygdala is implicated in extinction. Aim 1 will test the hypothesis that VNS modulates experience-dependent plasticity and the individual contributions of extinction training, VNS, and VNS paired with extinction on plasticity in an extinction-related neural pathway will be investigated. Aims 2 and 3 will examine whether VNS affects the same modulatory and synaptic systems that are involved in natural extinction. The proposed research will bring notoriety to a burgeoning center for neuroscience. NIH support will bring stability to student-led neuroscience research at a time when the university is rapidly growing by trial and error. Four exceptional undergraduate students, all neuroscience majors aspiring to careers in research and medicine and with GPAs over 3.7, will join two graduate students and two mentors to ask the basic question of how a potential therapy works.